Jupiter probe challenges planet-formation theories

Elements may have come from outside solar system

November 18, 1999|By NEW YORK TIMES NEWS SERVICE

A probe that plummeted into Jupiter's atmosphere late in 1995 found more than twice the concentration of volatile elements, such as argon and nitrogen, than was expected, raising questions about standard theories of how the planets formed.

The results, which a team of scientists led by Dr. Tobias Owen of the University of Hawaii is reporting in today's issue of Nature, suggest that at least some of the rocky, icy bits of dust and ice that crashed together to form Jupiter -- comet-like bodies called planetesimals -- must have originated under cooler conditions than prevail in the region of the solar system where the planet orbits now.

Otherwise the planetesimals would not have been cold enough to trap the volatile gases, which would have been dispersed among the other tenuous matter in interplanetary space.

The observations, some researchers said, could mean that some planetesimals rained down on the forming planet from the colder, outer reaches of the solar system that are farther from the warmth of the sun.

Other planetary scientists suggested that the results imply that the whirling disk of dust, gas and rock in the young solar system, called the solar nebula, simply shaded the sun's light more than expected in standard theories.

There is even the possibility that Jupiter formed as much as 10 times farther from the sun than it is now, and somehow lumbered inward to its present position. Most scientists dismissed that possibility as extremely unlikely, but said the results nevertheless posed a challenge for theories of planet formation.

"The specific pattern they find is puzzling," said Dr. Jonathan I. Lunine, a professor of planetary science at the University of Arizona, "and it seems to require that the planetesimals that carried these materials into Jupiter came from very, very cold regions."

That conclusion could mean that the planetesimals came from near where the more distant planets Uranus and Neptune are now, or from still farther out, said Dr. Alan P. Boss, an astrophysicist at the Carnegie Institution of Washington.

More likely, he said, is that the strong gravity of the forming planet pulled in objects from slightly outside its present orbit, and that the nebula was shaded more completely than earlier estimates had suggested.

"Now we have a new constraint saying, `Well, it had to be cooler yet,' " Boss said.